1. Field of the Invention
The present invention relates to liquid molding. Specifically, the present invention relates to active control of the liquid molding process and press during mold filling and curing.
2. Description of the Related Art
A brief overview of the techniques that currently dominate the production of liquid molded composites will be useful in demonstrating the benefits of the process of the present invention. Conventional processes that are most similar in capabilities to the present invention are: compression molding of Sheet Molding Compound (“SMC”), Resin Transfer Molding (“RTM”), and Structural Reaction Injection Molding (“SRIM”).
The SMC process typically starts with a sheet of unsaturated polyester resin filled with various thickeners and reinforced with chopped glass. The sheets are cut and placed in a heated tool and compressed at temperatures ranging from 140–200° C. (280–390° F.) and pressures ranging from 7–14 MPa (1000–2000 psi) down to as little as 1.4 MPa (200 psi) for new low pressure formulations. As the sheets are heated and compressed, the viscosity drops and the material flows along the contours of the mold, typically curing in about 2 minutes. The SMC process differs from liquid molding techniques in that the resin and fibers are premixed in a separate operation. The primary advantage of the SMC process is that a preform does not have to be constructed. The primary disadvantages of the SMC process are its relatively long cycle times and low strength to weight ratios of the resulting parts.
In a typical RTM process, a fiber preform is placed in matched tooling, compressed, and low viscosity statically mixed reactants are injected into the cavity through single or multiple ports at pressures ranging from vacuum driven to 1.4 MPa (200 psi). As the resin front progresses, it forces out any entrapped air through one or more vents placed in the matched tooling. After the resin begins to flow out of the vents, the vents are closed and the part is allowed to cure, typically from 4 to 30 minutes, depending on the part size, part geometry, the number and placement of ports, and the specific resin system. A diagram of the RTM process appears in FIG. 1 below. In general, tooling and energy costs are low for the RTM process, but its high cycle times reduce manufacturing volumes. The main drawback of the RTM process, as a mass production technique, is its fill time.
FIG. 2 shows that the SRIM process is similar to the RTM process, with the primary exceptions being that the resin is impingement mixed at very high pressures 100 MPa (1000 bar) and then injected into a heated tool at pressures ranging from 0.5–1.7 MPa (70–200 psi). The resin systems used in the SRIM process react very quickly and can cure in as little as 45 seconds. To allow mold filling before the resin gels, the preforms usually do not exceed a 30% volume fraction. The SRIM process has generally been employed with better quality molds, injection equipment, and process control than available for the RTM process. These factors have led to a distinction between the two processes; the RTM process as a slow, inexpensive technique producing very strong parts vs. the SRIM process as a more sophisticated and expensive method for the very rapid production of non-structural components. In reality, the differences between the processes are slight. The SRIM process is simply the RTM process using reaction injection molding, typically in a higher quality, heated mold.
FIG. 3 schematically shows the progression that the resin front takes as it infuses a part in the RTM and SRIM processes. Typical times for injection, for example, into a preform with a 40% fiber volume fraction, are noted. If the resin is forced too quickly through the part, air bubbles may be trapped or the fibers of the preform may be displaced, degrading the properties of the part. Alternatively, changing the flow path, for instance, by infusing the resin from the center of the part out to the edges, is difficult and may result in nonuniform properties. In general, the resin flow path is the limiting factor in reducing the cycle time of these techniques.
In “Study on Compression Transfer Molding (CTM)” published in the Journal of Composite Materials, Vol. 25, No. 16,1995, Young and Chiu describe the CTM goal to be “impregnation through the thickness direction.” In their test apparatus they left the mold halves slightly open and injected resin into the cavity at various pressures and recorded filling time. If the mold was not opened enough, the fiber preform merely decompressed somewhat, still impeding the flow of the resin. Once the proper opening distance was determined, mold fill times dropped by 37–46% over RTM at the same injection pressure. The proposed mechanism for this was a channel flow between the preform and mold. The mold is then closed, completing infusion in the thickness direction very quickly with minimum disturbance of the fibers. The strength and modulus of the completed part was shown to be the same as an RTM part. The limitation of CTM is that the preform is not rigidly held in place during injection and does not create a true open channel for resin to flow through, limiting the maximum rate at which injection can occur. The lowered flow resistance RTM process is still very helpful, especially when infusing very large planar parts like automobile body panels. It should also be noted that if very high fiber volume fractions are sought, the amount of resin injected into the mold is not enough to distribute throughout the mold, and compression times must be lengthened to allow time for some of the resin to flow through the in-plane direction. The Dodge Viper used a version of CTM called Injection Compression System (ICS) for many of its components, but as yearly volumes were low, cycle times could be as long as 15 minutes. Part finish was not perfect, but this may have been a problem with other aspects of the process such as resin system, release agents, etc.
Another innovative process that attempts to infuse primarily through the thickness direction is the patented Seemann Composite Resin Infusion Molding Process (“SCRIMP”). This is a variation on RTM with vacuum assist under a flexible tool, so only one hard mold surface is required. The resin is channeled through a high permeability “distribution medium” placed between the tool surfaces and the preform. A vacuum is pulled on the preform and the resin is introduced into and quickly distributed through the medium. The resin then infuses into the part through the thickness direction, creating a very uniform, high volume fraction part. A porous peel ply is placed between the distribution medium and the preform so that it can be removed and disposed of. The process has proven extremely popular for infusing huge, planar parts like large boat hulls and railway cars. SCRIMP works well, but as a vacuum driven process, it is too slow and also generates too much scrap to be considered for mass production. Seemann has another patent (U.S. Pat. No. 5,601,852) which details a variation of the through thickness approach used in SCRIMP that employs physical channels in a flexible, molded outer tool surface. The tool can, unlike the vacuum bag distribution medium, be quickly cleaned and reused, but will still not generate the cycle times or scrap levels required for mass production.
Another interesting RTM-like system developed by James et al. of the Northrop Corporation is detailed in U.S. Pat. No. 5,204,042. This process attempts to avoid the maximum fiber volume limitation of RTM, quoted as “50–60% by weight” (presumably for glass) by sandwiching an elastomeric pad made of Dow Silastic® E silicon rubber between mold surfaces. The pad expands when heated, compressing the fiber at up to “75–80% by weight.” The part is infused under lower compaction and then compresses tremendously when heated for curing. This speeds infusion while providing a very high quality part. Like SCRIMP, only one tooled mold surface is needed, but a very rigid upper mold section is required.
The trend in RTM-like processes is toward through-thickness infusion. CTM, SCRIMP and other variants achieve superior results to traditional liquid molding with their modifications. But each must trade something for its gains. CTM decreases mold filling times, but is still sensitive to the volume fraction of the preforms. SCRIMP works well even with high volume fractions, but is limited in speed by using vacuum pressure to drive infusion. The Northrop process delivers improved mold filling and very high volume fraction, but is still limited by its in-plane infusion path.
An important factor in many modern processing machines is the amount of control that can be exercised over the process. The advent of modern computer technology has allowed the development of remote input/output systems that communicate over one wire and have very sophisticated programming and diagnostic tools. These systems have been finding their way into more and more industrial applications and will someday displace all current PLC based controllers as well as introducing sophisticated computer control where it has never been before. Although there are many different protocols in the market, the industrial control market and the personal computer market have been getting together to create some software and communication standards. Even today there is a vast range of hardware and software solutions from basic on/off control of a motor to running entire plants.
Each of the known processes have limitations that prevent them from being used to produce structures that exploit the full potential of composite material design. The SMC process has a very low cycle time, but it is restricted to relatively low fiber volume fractions with short fiber lengths, reducing the specific strength of the part. The RTM process can operate with higher fiber volume fraction preforms, but the resin typically must flow through the plane of the preform and the higher the fiber volume fraction, the lower the permeability, and the more difficult and time consuming the resin flow step becomes. Variations of the RTM process have attempted to solve the resin flow problem by using multiple, staged injection ports, but process control can be very difficult and each mold must be painstakingly optimized. In the SRIM process the flow rates are even higher to allow the use of faster curing impingement mixed resins, such as polyurethanes. The required faster flow rates limit the maximum fiber volume fraction to a level well below the level for optimizing the properties of the part. These known methods have achieved production-ready cycle times, but the trade-off for this is a low fiber volume fraction, resulting in a part with extra resin that adds unnecessary weight and cost.
The ideal liquid molding process is one which: (1) can easily infuse very high
fiber volume fraction preforms thereby maximizing the physical properties of the resulting part and minimizing the cost of resins; (2) can offer very low cycle times thereby enabling large volume productions as cheaply as possible; (3) can use inexpensive tooling and process equipment; and (4) can quickly, easily, and cost-effectively accommodate small production runs.